An example of such a transponder has been described in the European patent application EP-A-0 172 445.
Its two antenna elements are each formed by a thin metallic plate having for instance a substantially rectangular form.
The passive electronic component which couples the two antenna elements is for instance a diode having a quadratic current-voltage characteristic such as the diode sold under the serial number 5 082-2835 by the Hewlett-Packard company.
Such a transponder is in particular intended to be borne by a person running the risk of being buried under an avalanche. If such an accident should happen, the person may be located with the help of a suitable detection apparatus brought into proximity therewith and including a transmitter producing electromagnetic radiation having a wavelength substantially equal to double the total length of the transponder.
When it is subjected to such electromagnetic radiation, the transponder emits a second electromagnetic radiation having a wavelength substantially equal to its total length and thus to half the wavelength of the radiation produced by the transmitter. This second electromagnetic radiation may be captured by a suitable receiver which is likewise included in the detection apparatus, thus enabling the location of the transponder and the person carrying it.
The various components of the transponder form an oscillating circuit, the resonance frequency of which must be as close as possible to the frequency of the electromagnetic radiation emitted by the detection apparatus in order that the yield of the transponder, i.e. the ratio of the quantity of energy which it emits to that which it receives, be high.
But the metallic plates forming the elements of the transponder antenna give rise, with the body of the person bearing it, to a stray capacitor, the capacitance of which influences the resonance frequency of the oscillating circuit mentioned hereinabove.
It is not possible to compensate for the effect of that stray capacitor by a proper dimensioning of the other components of the transponder since its capacitance depends directly from the distance separating such transponder from the body of the person bearing it which distance is variable in accordance with whether the transponder is worn over one or several layers of clothing.
The resonance frequency of the oscillating circuit formed by the elements of the transponder is thus practically never equal to the electromagnetic radiation, frequency emitted by the detection apparatus and the gain of the transponder is thus almost never optimum.
Furthermore, the stray capacitor transmits a portion of the energy received by the transponder, as well as a portion of the energy which it emits, to the body of the person bearing it, this bringing about likewise a diminishing of the gain of the transponder.
A purpose of the invention is to provide a transponder of the type which has been described hereinabove but which provides a higher gain than the known transponder.